Given that Donalbain doesn’t get many lines in the play, it seems safe to assume that you are referring to the interaction between the two brothers at the end of act 2, scene 3. Macbeth has just found the dead body of their father, the king, and Macbeth and Lady Macbeth are playing their parts, acting as though they are shocked and saddened by the news of Duncan’s death.
Donalbain, the younger prince, suggests immediately to his older brother that they both flee, as they need time to think and plan their next move. They have had no chance even to mourn their father, and yet they must be concerned about someone coming to do the same to them as has been done to him. Malcolm says that the brothers ought “not consort” with the nobles, as “to show an unfelt sorrow is an office / Which the false man does easy” (2.3.161). They know that it was likely one of the men who was just expressing sadness over the king’s death who is actually guilty of the murder. Malcolm says that he will flee to England, and Donalbain says that he will go to Ireland because their “separated fortune / Shall keep [them] both the safer” (2.3.263–264).
Donalbain also, quite presciently, says that, wherever the princes are, “There’s daggers in men’s smiles. The near in blood, / The nearer bloody” (2.3.265–266). These lines in particular foreshadow the future treachery of Macbeth. He may have felt regretful and scared immediately after the murder, but he is certainly “near” to the princes in blood, as cousin to their father, and he will turn out to be far more violent and unscrupulous than anyone could have previously imagined.